The present invention relates to hypodermic syringes, particularly sterilized prefilled or prefillable syringes, having a needle shield or sheath providing reduced needle penetration force, a method of making a sterilized syringe assembly and an improved multibeveled needle tip configuration.
A hypodermic syringe typically includes a generally tubular barrel portion, which may be formed of glass or plastic, a plunger having a stopper typically formed of an elastomeric material, such as rubber or synthetic rubber, and a needle cannula typically formed from an elongated tube having a fluid-conducting lumen. Such syringes may be prefilled with a medicament, drug or vaccine which require a shield or sheath enclosing the sharp end of the needle cannula typically formed of rubber or synthetic rubber. A needle shield includes an open end, a closed end, and a needle passage through the open end which receives the sharp end of the syringe needle cannula. As will be understood, hypodermic syringes must be sterilized prior to use by the healthcare worker or patient and such syringes are typically sterilized by the manufacturer and generally sealed in a plastic container ready for use.
A preferred method of sterilizing hypodermic syringes, particularly prefillable or prefilled syringes, is to xe2x80x9cimmersexe2x80x9d the syringe assembly in a sterilizing gas, such as ethylene oxide. Although there are several industry recognized methods of gas sterilization, such methods depend upon permeation of the sterilization gas into the passage of the needle shield to sterilize the syringe needle cannula. However, natural and synthetic rubber and vulcanizate thermoplastic elastomers are characterized as having a low gas permeability. Further, ethylene oxide gas, which is commonly used for gas sterilization. Alternatively, steam sterilization may also be used, but is generally limited to subsequent or xe2x80x9cterminalxe2x80x9d sterilization. As used herein, xe2x80x9csterilization gasxe2x80x9d may be any gas used for sterilization, including ethylene oxide and steam. Therefore, the cycle time required for gas sterilization is relatively long. That is, the syringe is first immersed in the sterilization gas for a time sufficient for the gas to sterilize the syringe, including the needle cannula. Following sterilization, the sterilized syringes are xe2x80x9cquarantinedxe2x80x9d for a time sufficient for the sterilization gas to escape, including any residual gas trapped in the needle shield. Thus, the sterilization cycle time is dependent in part upon how easily the gas penetrates through the needle shield during sterilization and removal of the gas from the syringe assembly. Tests are conducted to confirm that the sterilized syringe assemblies contain only minute traces of residual ethylene oxide or water in steam sterilization prior to release for distribution or sale.
A particular concern with the design of syringes is reduction of the needle cannula penetration force and patient comfort. The distal end or point of the needle cannula is typically provided with a tip geometry for piercing a patient""s epidermis, flesh or tissue to deliver a fluid medicament, drug or vaccine stored or held in the syringe barrel. A healthcare worker or patient may also employ the syringe needle cannula to pierce an elastomeric septum or stopper of a vessel, such as a vial, to reconstitute dry or powdered medicament, drug or vaccine or to aspirate a liquid medicament, drug or vaccine contained in the vial.
Various considerations must be made when designing a syringe. For example, it is obviously desirable to minimize the needle cannula penetration force necessary for urging the needle cannula point or tip through the epidermis and flesh of the patient. It is generally believed that by reducing the needle cannula penetration force, the patient will perceive less pain. Another consideration in designing needle cannula point geometry is to prevent or minimize xe2x80x9ccoring.xe2x80x9d Coring, as those skilled in this art understand, results when a portion of the material through which the needle cannula has penetrated becomes lodged in the lumen adjacent the needle cannula tip.
Various attempts have been made to reduce the required penetration force of syringe needle cannulas and reduce coring as discussed more fully in the above-referenced co-pending application. These efforts have been primarily directed to improving the design of the needle cannula tip by providing facets or bevels, for example, to reduce the required penetration force. Other attempts have been made to minimize the required penetration force by minimizing coring. However, these efforts have not been as successful as desired. Further, various efforts have been made to improve syringe needle cannula shields or sheaths, particularly for prefilled hypodermic syringes. Such improvements generally relate to protecting the needle cannula and preventing inadvertent coring of the needle shield by the needle cannula as disclosed, for example, in U.S. Pat. No. 4,964,866 assigned to the assignee of the present application, the disclosure of which is incorporated herein by reference. Further efforts have been made in the design of needle shields or syringes to reduce the gas sterilization cycle time by providing non-linear channels in the needle cannula shield which permit entry and egress of the sterilization gas while preventing entry of microorganisms.
However, no one has recognized the inter-relation between the selection of the material from which the needle shield is formed and the required penetration force of the needle cannula. The present invention relates to an improved five-beveled point geometry for a hypodermic needle and a needle shield which reduces the penetration force of the needle cannula. It is also believed that the improved needle shield will reduce gas sterilization cycle time.
The syringe assembly of this invention utilizes an improved five-bevel needle configuration which reduces penetration force and a needle cannula shield or sheath formed of a styrene block thermoplastic elastomer which maintains the sharpness of the needle cannula during application, sterilization and removal of the shield, and may reduce the cycle time of gas sterilization. As described above, the improved five-beveled needle cannula configuration and needle shield may be utilized with any conventional injection device, including a conventional prefilled hypodermic syringe, and the improved needle shield or sheath of this invention has further advantages when the syringe assembly is gas sterilized. A conventional syringe assembly includes a generally tubular barrel, typically made of glass, but which may also be formed from various polymers, a needle cannula fixed to the tip portion of the barrel having a lumen therethrough in fluid communication with the interior of the barrel portion and syringes. Prefillable and prefilled syringes include a needle shield having an open end and a needle passage through the open end which receives the sharp distal end of the needle cannula to protect the needle cannula and prevent loss of fluid in the syringe barrel. The needle cannula is typically formed of stainless steel, such as AISI 304, and the needle cannula is generally coated with a lubricant, such as a silicone oil. U.S. Pat. No. 5,911,711 assigned to the assignee of this application discloses preferred needle lubricants. The needle shield or sheath is typically formed of a natural or synthetic rubber generally including a significant amount of a filler to improve the mechanical properties and reduce cost. More recently, with the advent of thermoplastic elastomers replacing rubber and synthetic rubber polymers in various applications, the prior art has suggested the use of thermoplastic elastomers for syringe tip shields and tip caps. However, as set forth below, most thermoplastic elastomers provide little if any advantage over natural or synthetic rubber and vulcanizate thermoplastic elastomers suffer other disadvantages, including shrinkage during molding, lack of dimensional stability and coring. There is, therefore, a need for an improved needle cannula point configuration which reduces penetration force and a needle shield which protects and maintains the sharpness of the needle cannula point.
It is believed by the inventors that a primary reason that a patient experiences pain when a needle cannula penetrates the skin or flesh of the patient, the needle point catches on the skin or flesh as the needle penetrates. One cause of a needle point catching on the skin or flesh is believed to be due to the height of the xe2x80x9cintersectxe2x80x9d established at the transition between differing bevels forming the needle point. It is believed that if this transition between differing bevels forming the needle point is less pronounced, the height of the intersects would be reduced. The effect of reducing the heights of the transitions would be to approximate, from a series of bevels forming the cannula needle point, a more continuous, unitary bevel face. The resulting continuing bevel point would thus require less penetration force in entering a patient""s skin and flesh. By reducing penetration force, it is believed that the patient will also experience less pain.
Accordingly, one aspect of this invention relates to a multi-beveled needle point, reducing the heights of the intersects created between merging bevels that results in a more continuous bevel face. As described above, a needle cannula has a central lumen defining an axis through the needle cannula. The multi-beveled cannula needle point defines an opening to the lumen for the passage of fluids between a medical delivery device, such as a syringe, and a patient or vessel. The multi-beveled point preferably includes a primary bevel, a pair of tip bevels, and a pair of middle bevels. Each of the middle bevels are contiguous with the primary bevel, and meet a respective one of the tip bevels at an intersect. The primary bevel is formed or otherwise provided on the cannula by inclining the central axis of the needle cannula to a first planar angle respective of a reference plane.
The needle point, formed of five distinct bevels, displays reduced height intersects, resulting in a more continuous bevel face about the opening. It is believed that by providing a series of five distinct bevels, the needle point is lengthened over the needle points conventionally in use, and owing to the reduced height intervals, results in an effective outer diameter at the needle point less than the outer diameter of the needle points currently in use, all of which contribute to reduced needle penetration force.
The needle shield of this invention is formed of a styrene block thermoplastic elastomer, most preferably a styrene block(polyethylene/butylene) thermoplastic elastomer having a Shore A hardness of between 30 and 90 or more preferably between 45 and 65. The needle shield includes an open end and a passage through the open end configured to receive the needle cannula and preferably encloses the entire needle cannula and a portion of the syringe tip to fully protect the cannula and prevent entry of microorganisms following sterilization. In the most preferred embodiment, the internal passage includes an integral annular rib, adjacent the open end, which assures retention of the needle shield on the syringe tip portion and the needle shield is preferably enclosed by a rigid cover or shield.
As discussed in more detail hereinbelow, the needle shield of this invention has several unanticipated and unexpected advantages over needle shields formed of natural or synthetic rubber or conventional vulcanizate thermoplastic elastomers. First, extensive bench and clinical testing has established that a needle cannula shield formed of a styrene block thermoplastic elastomer, particularly a styrene block poly(ethylene/butylene) thermoplastic elastomer maintains the sharpness of the needle cannula, particularly the needle cannula point, during application and removal of the needle shield as compared to natural rubber or synthetic rubber needle shields and conventional vulcanizate thermoplastic elastomers. This improvement results in reduced penetration force which is now believed to result from at least two factors which were discovered during clinical trials and bench testing. First, the needle cannula of a hypodermic syringe is conventionally coated with a medical grade lubricant, such as a silicone oil lubricant, to reduce penetration force. It is now believed that a conventional needle shield wipes away the lubricant on the needle cannula, particularly at the needle point, which is important to reduction of penetration force. As set forth above, a conventional needle shield includes a passage configured to receive the needle cannula and most preferably includes a small bore which closely receives the needle point. Thus, the lubricant may be wiped away during the receipt and removal of the needle shield on the needle cannula. This was confirmed by electron microscopic examination of the needle cannula following insertion of the needle cannula in the needle shield and removal of the needle shield from the needle cannula. The lubricant may also be absorbed by the needle shield, particularly natural and synthetic rubber needle shields having a high filler content. Second, needle shields formed of natural or synthetic rubber having a high filler content are abrasive, which may result in microabrasion of the needle point reducing the sharpness of the needle. Regardless of the cause, however, bench and clinical testing has established that use of a needle shield formed of a styrene block poly(ethylene/butylene) thermoplastic elastomer results in less penetration force and improved sharpness as perceived by healthcare workers making the injections. This improvement was unexpected.
Another potential advantage of a needle shield formed of a styrene block thermoplastic elastomer, particularly a styrene block poly(ethylene/butylene) thermoplastic elastomer, is reduced at sterilization cycle time. As set forth above, syringe assemblies are conventionally sterilized with ethylene oxide gas, which is toxic. During sterilization, the syringe assembly is flooded or xe2x80x9cimmersedxe2x80x9d in the ethylene oxide gas or other sterilization gas including steam by one of several known methods. As will be understood, the sterilization gas must xe2x80x9cpenetratexe2x80x9d the needle shield to sterilize the needle cannula. However, rubber and conventional vulcanizate thermoplastic elastomers are characterized as having a low gas permeability, resulting in slow transmission of the sterilization gas into the passage in the needle shield containing the needle cannula. Further, because ethylene oxide gas is toxic, all of the sterilization gas must be removed from the needle shield before packaging. Again, because of the slow transmission of the sterilization gas out through the needle shield, the syringe assembly is quarantined until substantially all of the sterilization gas permeates out through the shield. Unexpectedly, a styrene block poly(ethylene butylene) thermoplastic elastomer has a relatively high gas permeability to ethylene oxide gas as established by testing for residual gas in the shield following sterilization.
The most preferred embodiment of the needle shield for a syringe needle cannula of this invention is formed of a styrene block poly(ethylene/butylene) thermoplastic elastomer having a Shore hardness of between 45 and 65. The most preferred composition for the needle shield of this invention also includes about one to three percent colorant which contains carbon black to improve structural integrity and reduce coring.
Other advantages and meritorious features of the syringe assembly of this invention will be more fully understood from the following description of the preferred embodiments, the appended claims, and the drawings, a brief description of which follows.